Example:

Workbook workbook = new Workbook();
workbook.getWorksheets().add();
workbook.getWorksheets().add();
Cell cellInPage1 = workbook.getWorksheets().get(0).getCells().get("A1");
Cell cellInPage2 = workbook.getWorksheets().get(1).getCells().get("A1");
Cell cellInPage3 = workbook.getWorksheets().get(2).getCells().get("A1");
cellInPage1.putValue("page1");
cellInPage2.putValue("page2");
cellInPage3.putValue("page3");

PdfBookmarkEntry pbeRoot = new PdfBookmarkEntry();
pbeRoot.setText("root");  // if pbeRoot.Text = null, all children of pbeRoot will be inserted on the top level in the bookmark.
pbeRoot.setDestination(cellInPage1);
pbeRoot.setSubEntry(new ArrayList());
pbeRoot.setOpen(false);

PdfBookmarkEntry subPbe1 = new PdfBookmarkEntry();
subPbe1.setText("section1");
subPbe1.setDestination(cellInPage2);

PdfBookmarkEntry subPbe2 = new PdfBookmarkEntry();
subPbe2.setText("section2");
subPbe2.setDestination(cellInPage3);

pbeRoot.getSubEntry().add(subPbe1);
pbeRoot.getSubEntry().add(subPbe2);

PdfSaveOptions saveOptions = new PdfSaveOptions();
saveOptions.setBookmark(pbeRoot);
workbook.save("output_bookmark.pdf", saveOptions);